Uranium solution absorption spectra

The majority of the photochemical studies with actinide ions have been carried out with the uranyl (UC ion. This ion is yellow in color both in the solid and solution states. The early photochemistry of this ion has been reviewed. " Excitation of this ion results in an LMCT absorption that involves a transition from an essentially nonbonding 7r-orbital on oxygen into an empty 5/orbital on uranium. This LMCT assignment is that given to the weak visible bands in the absorption spectrum at 500 nm and 360 nm. The absorption spectrum also shows a series of bands of increasing intensity to higher energy. The positions of the absorption bands of are very sensitive to both temperature and the chemical environment... 

For thorium there are only estimates of the corresponding potential. An early estimate, of -2.4 V, was based on a relation between this quantity and the frequency of the first electron transfer absorption band in the UV spectrum of an aqueous thorium perchlorate solution (9). However, the spectral measurements did not quite reach the absorption maximum, and the necessary extrapolation introduced some uncertainty. Another value, -3.6 V, was based on the RESPET treatment of J0rgensen (10,11). The adjustable parameters in the RESPET equation were fixed using experimental values for other actinide elements (12). This method yields a value of -0.69 V for U(IV)/(III). Another rather simple method correlates this potential with the number of 5/"electrons for the element and gives -3.41 V for thorium and -0.54 V for uranium (13). A more sophisticated estimate (14), using a method proposed by Nugent et al. (12) (described later), gave -3.8 V for thorium. 

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